1. Field of the Invention
The present invention is related to a measuring apparatus and a measuring method, and more particularly to a measuring apparatus and a measuring method for improving the reliability of accurate distance measurement when a dual frequency continuous wave (CW) method is used.
2. Background Art
One example of a conventional sensor uses the dual frequency CW method to measures a relative speed and a distance between a vehicle to which the sensor is attached and another vehicle (see, e.g., Japanese Patent No. 3,203,600 and JP-A-2004-69693). This dual frequency CW method sensor detects a frequency of a Doppler signal and a phase of the Doppler signal produced for a received carrier wave, and uses the detected frequency and phase to measure the relative speed and distance between the vehicle to which the sensor is attached and another vehicle.
In the dual frequency CW method sensor, the distance cannot be measured when the relative speed is equal to zero.
In addition, when a relatively high-frequency noise is superposed on the carrier wave received by the dual frequency CW method sensor, the phase of the Doppler frequency varies due to the presence of the noise. These variations of the phase are small as viewed individually, but can produce large distance errors as the carrier wave also has a high frequency. Thus, in the dual frequency CW method sensor, distance measurement accuracy can decrease when relatively high-frequency noise is superposed on the carrier wave.
Accordingly, reliability of accurate distance measurement using the dual frequency CW method is low.
The conventional devices are characterized by a combination of a modulation method used in the dual frequency CW method and another modulation method (e.g., frequency pulse CW method or FM-CW method). The frequency pulse CW method measures a distance based on the delay time of a pulse. Thus, distance accuracy decreases when high-frequency noise is superposed on the carrier wave. The FM-CW method calculates a distance based on variation of frequency, that is, deviation of carrier wave frequency. Thus, distance accuracy is similarly affected by high-frequency noise superposed on the carrier wave.